1. Field of the Invention
The present invention relates to novel diaphragms comprising thermoplastic fibers adapted for use in electrolytic cells, to the coupling of such diaphragms with a cathode component of such cells and to a process for producing such diaphragms and coupling such diaphragms with a cathode component.
This invention especially relates to improved diaphragms produced via a wet route, based on thermoplastic fibers and devoid of asbestos fibers, and adapted for use in chlorine/soda electrolysis cells.
2. Description of the Prior
Asbestos fibers have long been employed as a conventional material for producing the diaphragms used in electrolytic cells. These diaphragms are fabricated by depositing asbestos fibers contained in an aqueous mash onto a cathode which is permeable to the electrolytes, deposition operation being carried out under vacuum. Thus, French Patent No. 2,213,805 describes preparing microporous separators by depositing a layer of asbestos, said layer being consolidated by a fluoropolymer. The porosity of such a layer can be better controlled by adding a pore-forming agent according to the technique described in French Patent No. 2,229,739.
As is well known to this art, such preparation of microporous separators by depositing, under vacuum, an aqueous mash containing fibers and a binder presents a very great advantage, both from a technological standpoint, as well as from an economic standpoint. However, the quality of the separators thus produced is not fully satisfactory because of the necessity to use asbestos fibers. Indeed, notwithstanding the hazards associated with the handling of asbestos, which is dangerous to human health, the insufficient chemical stability inherent in asbestos presents various disadvantages such as excessively short useful life of the separators and difficulties in modifying the operating conditions of the electrolyzer, for example by increasing the electrical current.
In published European Patent Application No. 132,425 cathode components have been described, composite material produced by the coupling of an elementary cathode including a highly porous metal surface such as a metal grid having a mesh opening ranging from 20 .mu.m to 5 mm and of a sheet containing fibers and a binder, the coupling and the sheet resulting from the programmed suction under vacuum of a suspension containing essentially electrically conductive fibers and a fluoropolymer, directly through said elementary cathode, followed by drying, and then melting the binder. Such composite materials are adapted to themselves constitute the cathode of an electrolysis cell and may be coupled with a diaphragm, it being possible for the diaphragm to be manufactured directly by a wet route on the composite.
Various improvements have also been made, both to the composite materials themselves, as well as to the process for the manufacture thereof.
In published European Patent Application No. 214,066 materials are described, containing carbon fibers exhibiting a monodisperse length distribution, materials whose quality and properties are very appreciably improved, and this is reflected in a much more favorable performance/thickness relationship.
In published European Patent Application No. 296,076 electroactivated materials are described which contain an electrocatalytic agent uniformly distributed within their bulk mass, said agent being selected from among Raney metals and Raney alloys from which most of the easily removable metal(s) has (have) been removed.
The assemblage of proposed cathode components which ensure an appreciable distribution of the current is adapted for use in an electrolytic cell which will comprise a membrane or a diaphragm between the anode and cathode compartments. Additional technical details are described in the aforementioned European patent applications, hereby expressly incorporated by reference in respect of the construction of said cathode components.
Similarly, it has long been proposed to manufacture microporous separators based on thermoplastic materials using traditional methods of shaping plastics, such as kneading, molding, calendering and/or sintering.
Thus, French Patents Nos. 2,280,435 and 2,280,609 describe the manufacture of microporous separators based on fluoropolymers. However, the planar diaphragms thus obtained are difficult to install in cells whose cathode components are geometrically complex, and they are very difficult to wet. It has not been possible to satisfactorily develop these separators, the cost of manufacture of which remains relatively very high.
Furthermore, it is not possible to produce satisfactory microporous separators simply by substituting the asbestos fibers in the mashes intended for the preparation of diaphragms by deposition under vacuum with fibers of a material such as polytetrafluoroethylene (PTFE) which would be a most attractive route for preparing microporous separators, especially in the case of cells whose cathode components are geometrically complex. Indeed, the layer of PTFE fibers shrinks very greatly during the sintering, the uniformity and the fineness of the porosity are inadequate and the hydrophilicity of the layer of fibers is precarious.
More recently, it was proposed in U.S. Pat. No. 4,680,101 to prepare a modified diaphragm by impregnating a preformed diaphragm (matrix) with a solution, in an organic solvent, of a partially hydrolyzed metal alkoxide represented, before hydrolysis, by the formula M(OR).sub.4, in which M is titanium, zirconium, silicon or aluminum, and R is an alkyl radical having from 1 to 6 carbon atoms; the diaphragm impregnated in this manner is then heated to 90.degree.-150.degree. C. to crosslink the polymeric metal oxide.
The matrix itself is formed by vacuum filtration on a cathode of a mash containing polytetrafluoroethylene which is fibrillated, if appropriate, an ethanol solution of a perfluoro ion exchanger containing sulfonic acid groups, followed by drying the product under vacuum in an oven at 120.degree.-130.degree. C. The perfluoro ion exchanger imparts wettability to such a diaphragm.
After forming this matrix and, preferably, after impregnating the latter with a view to providing it with a polymeric metal oxide, it is suggested to incorporate in the already formed diaphragm an inorganic gel such as gels of magnesium, zirconium or titanium oxides or a zirconyl phosphate gel; such gels are said to be capable of reducing the permeability of such a diaphragm to liquids and of imparting ion exchange properties to said diaphragm.
These impregnated diaphragms, whose basic advantage is apparent, exhibit various disadvantages and in particular a lack of homogeneity and a permeability which is difficult to control.
Furthermore, the process of manufacture is characterized by a complexity which is difficult to reconcile with industrial requirements.